Cell Communication: AP Biology Unit 4

Direct Contact Signaling

Contact through cell junctions. Gap junctions in animals, or plasmodesmata in plants.

Cell to Cell Recognition

Another type of direct local signaling. Glycoproteins-we see this in the immune system.

Paracrine Signaling

Secreting cells discharge local regulator molecules to nearby target cells. Like growth factors to tell a cell to grow and divide.

Synaptic Signaling

Nerve cells release NT across a synapse, stimulating the target cell.

Long Distance Cell Communication-Hormones

-Specialized cells called endocrine cells secrete hormones into vessels. Hormones travel through the blood stream to target cells in the body.

-Plant hormones can move through vessels or through cells or even through the air as gas

Signal Transduction Pathway

Series of steps/reactions in a cell that are caused by a signal on the cell’s surface that triggers a specific cellular response.

3 Stages of Cell Signaling

1. When a cell encounters a signaling molecules it must be recognized by a receptor protein

2. The signal must be changed or transduced into another form inside the cell

3. The cell can respond. Signaling molecule=ligand

Ligand

A molecules that specifically bonds to another molecule (usually a larger one) and usually results in a shape change to the receptor protein.

Reception: Stage 1 of Cell Signaling

Signaling molecules from outside the cell bonds to a receptor protein.

1. The binding between a ligand and a receptor is highly specific. A change in the receptor protein is the first step in the transmission of a signal.

2. Receptors are found in two places

Intracellular Receptors

Intracellular receptors are found inside the plasma membrane in a cytoplasm or nucleus. Hydrophobic ligands would cross the membrane and bind to an intracellular receptor.

Ex. testosterone

Cell Surface Receptors

Bind to water soluble ligands.

Ex. epinephrine binds to surface liver cells receptors causing a cascade that leads to the conversion of glycogen into glucose.

G-Protein Coupled Receptors

Also known as GCPR’s

1. ligand binds to receptor causing a shape change in G-protein coupled receptor

2. Receptor is activated and binds to an activated G protein

3. Activated G protein binds to an enzyme causing ONE cellular response

4. Return to original shape for reuse

Ligand Gated Ion Channel Receptors

1. Ion channel is closed without ligand present

2. Binding of the ligand to the receptor on the ion channel opens the gate for ions to flow through from high to low concentration

3. Causes a change in concentration gradient leading to a cell response

4. Removal of the ligand causes the gate to close, prevents ions from moving, cellular response stops.

ex. nervous system

Transduction-Step 2

A step(s) to bring about a specific cell response like a shape change of a protein or gene activation.

1. usually more than one step-cascade

2. can amplify a signal and help the cell regulate it’s processes

3. protein kinase: Inactive protein kinases are phosphorylated and activated with the help of the previously activated protein. Ultimately the final protein goes through a shape change as a result.

4. protein phosophatase

Protein Kinase

An enzyme that transfers a P group from ATP to an inactive protein. Phosphorlates this protein.

Protein Phosphatase

Enzyme that removes a P group (dephosphorylate) from the active protein to turn off the signal tranduction pathyway.

Second Messengers

Small, non-protein, water-soluable molecules or ions that move through diffusion unlike an extracellular ligand attaching to the membrane receptor, (first messanger).

ex. cAMP and calcium ions

Example of a Second Messenger: cAMP from epinephrine signal

-cAMP comes from ATP breaking into 2 ATP and cAMP

-Triggered by epinephrine (first Messenger, ligand)

-G protein activated adenylyl cyclase

-Adenylyl cyclase catalyzes the reaction of ATP to cAMP and 2 P

-cAMP can broadcast to entries cytoplasm and activate Protein Kinase A

--Will continue to make more cAMP and activate more Protein Kinase A until inhibitors are activated

Nuclear Response (step 3)

Regulate protein synthesis by turning genes on or off by causing transcription.

Cytoplasmic Response (step 3)

Regulate the activity of proteins like enzymes. Open or close an ion channel which causes a change in cell metabolism.

Apoptosis

Programmed cell death. A normal part of vertebrae development. Happens constantly in nervous system and hands and feet.

-Example of a signal transduction pathway

Ways to stop a signal

-Block the receptor using a competitive signal, drug, or antibody

-Degrade the ligand

-Reuptake of the ligand-occurs in the nervous system

Types of Sensory Receptors

Classified by the type of stimulus that generates a response in the receptor. Respond to one of the four stimuli:

-Chemical (chemoreceptors)

-Temperature (thermoreceptors)

-Pressure (mechanoreceptors)

-Light (Photoreceptors)

Intensity of Sensation

The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor. The greater intensity of stimulation also will recruit more receptors into the receptive field.

Phototropism

The growth of shoots towards light, roots away from light

Darwin and Jensen’s Experiement

Tested phototrophic bending. Discovered that receptors for the plant are located in the tip of the shoot. Tip of the shoot and base must be able to communicate with each other, (tropism).

Responses to stimuli other than light

-Gravity-Gravitropism: roots grow downward and stems grow upward

-Touch-Trigmotropism: Vines wrap around objects when they touch them.

-Water-Hydrotropism: Roots grow toward moisture

Positive Feedback

Responses are amplified and the stimulus is further activated. Creates a reaction further away from homeostasis. Variable initiating the response is moved further away from the initial set point.

Negative Feedback

Drives a reaction closer to homeostasis. Stimulus is decreases and system is disrupted.

Parts of a chromosome

Sister chromatids are Identical chromosomes with the same function. The centromere links the sister chromatids together. Spindles attach to the chromosomes.

G0, G1, S, G2, and M

G0-Non-dividing state, occurs when cell doesn’t pass G1 checkpoint.

G1-Cells are stopped and must recieves signals to move on to S. Growth.

S-DNA replication.

G2-Makes sure DNA has been replicated correctly during S before moving on to mitosis.

M-Makes sure spindle fibers are attached correctly and chromomes move to the right locations.

Prophase

-First step

Chromatin condense into chromosomes. Chromosomes are attached by the centromeres. Nuclear membrane begins to break down. Centrosomes begin to separate and move to opposite ends of the cell. Spindle fibers form.

Prometaphase

-Occurs before metaphase

Condensed chromosomes start to organize in the middle of the cell

Metaphase

-step 2

Centrosomes/centrioles are at opposite ends of the cell. Sister chromotids line up in the center of the cell on the metaphase plate.

Anaphase

-step 3

Chromatids are pulled apart at the centromere. Spindle fibers pull chromatids towards opposite ends of the cell.

Telophase

-step 4

Chromosomes reach opposite ends of the cell. New nuclear membranes form around them. Chromosomes uncoil into chromatin.

Cytokinesis

Cytoplasm completely divides into 2 Identical daughter cells. In animal cells, the cleavage furrow pinches in to form the 2 cells. In plant cells, the cell plate becomes the cell wall and 2 cells are formed.

Cyclin

Proteins that activate CDKs. They also activate Protein kinases. Cyclin concentration increases during the S and G2 phase, there are no cyclins during the G1 phase. Control when the cell moves from one phase to another. Think of them as on and off switches

CDKs (Cyclin Dependent Kinases)

Enzymes that add phosphate groups to other proteins. Always present in the cell, but inactive until a Cyclin binds to them which activates them. When a Cyclin binds to a CDK, it triggers progression to the next cell cycle phase. They are the workers.

P53

Operates to suppress cell division. It can activate DNA repair proteins when DNA has gone through damage. It can stop growth by holding the cell cycle at the G1/S phase on DNA damage recognition. If it holds it here for awhile, DNA repair proteins can have time to fix damage and the cell cycle will continue. It can also initiate apoptosis.

MPF

Phosphorylates proteins, initiating mitosis. Phosphorylation on the nuclear membrane yields breakdown of the membrane. This may help to signal chromosomes to condense. MPF is switched off in anaphase and the attached Cyclin breaks down, inactivating the CDK.